It is generally well known in the art that passenger transit railway vehicles, such as subway cars and the like, are provided with individual brake cylinder units that apply a braking force onto the individual transit vehicle wheels. Such braking force may be applied either against the rolling face surface or edge surface of the wheel, or against one or both sides of a metallic brake disk in rolling motion with the wheel. Most such brake cylinder units comprise a rather heavy body structure rigidly secured to the vehicle under-structure adjacent to a wheel, which houses a brake piston reciprocally mounted within a receiving cavity. The unit is activated pneumatically, hydraulically, or through a combination hydraulic-pneumatic system, to drive a portion of the brake piston from the cavity, and thereby drive one or more brake shoes against the associated wheel. Depending on the brake design, the advancing outer end of the brake piston will directly or indirectly drive one or more brake shoes against a rolling wheel surface, or else activate linkage to cause one or more disk brake shoes to press on one or both sides of a rotating brake disk.
Such brake cylinders units are normally provided with some sort of slack adjusting means to maintain a predetermined clearance between the brake shoe or shoes and the wheel or disk, so that the brake piston is adjusted each time the vehicle brakes are applied. Indeed, many such slack adjusters have been disclosed in the prior art. Such prior art slack adjusters, however, are normally rather complicated, expensive to produce, and are of rather limited reliability.
In one such prior art slack adjuster, the brake piston is mounted for reciprocal motion on a support rod, with the slack adjuster also slidably attached to the support rod, such as to define an adjustable starting point, and return point for the reciprocal motion of the brake piston. Such a slack adjuster comprises a rod gripping means designed to tightly engage the support rod so that it is not readily slidable on the support rod, but is slidable with the application of fluid pressure forces applied by such brake cylinder.
A spring loaded compression member is also provided which is positioned to engage a second side of the rod gripping means, and is designed to be compressed between a flange on the brake piston and the rod gripping means when the brake piston is advanced in response to application of the brakes. As a result of such spring loaded compression, the compression member will return the brake piston into engagement with the rod gripping means when the brake cylinder is deactivated; i.e., when the brake fluid pressure is released. Each time the brakes are applied, the increase in fluid pressure will cause the brake piston to be partially advanced from the piston receiving cavity, whereby the spring loaded compression member is compressed, so that it will function to return the brake piston to a position against the rod gripping means when the brakes are released. The spring loaded compression member must have a limited degree of compressibility, however, so that the extent of its compressibility is fully utilized with each application of the brake forces. Then, as the brake shoe or shoes wear away, and the brake piston is forced to be moved further out with respect to the support rod, the full extent of compressibility of the spring loaded compression member is utilized before the brake piston reaches its maximum extended position, so that the fully compressed spring loaded compression member cannot be further compressed, and as a result the rod gripping means is incrimentally pushed outward along the support rod with each application of the brake, to thereby maintain a constant slack in the system, and accordingly a substantially constant clearance between the brake shoe or shoes and wheel or disk.
While a number of different concepts have been utilized to fashion a suitable rod gripping means, none have been particularly ideal, as all have been prone to relatively short life, in quickly failing to maintain the desired clearance between brake shoe or shoes and wheel or brake disk, and in being rather costly to fabricate. Specifically, as a result of wear on the various components and cylinder walls, such components may fail to grip as intended, thereby failing to provide the required shoe to wheel clearance as intended. In the manufacture of these slack adjusting components, some tolerances are so critical, that component parts must be sorted and matched in order to function properly and meet code tests.